The present invention relates to a chopped strand mat and a thermoplastic sheet, both of which are suited for the production of molded articles having improved strength, rigidity and impact resistance as well as attractive colors.
The glass fiber-reinforced thermoplastic resin is generally manufactured by adding short glass fibers to a thermoplastic resin (hereinafter referred to briefly as TP) and melt-compounding them to achieve a uniform dispersion of glass fibers in the TP. Shaped articles manufactured from the resulting compound by injection-molding are used widely as electric/electronic parts, automotive components and so on. However, since those injection-molded articles have anisotropy in strength and are inadequate in impact resistance, they have not been used as structural parts. Intended to obviate these disadvantages, the thermoplastic sheet manufactured by laying up TP and an isotropic glass fiber mat and pressing the combination under heating have been proposed in Japanese Patent Publication No. 54-36193 and Japanese Kokai Patent Publication No. 55-152058. The production of such sheet essentially requires a step of holding the glass fiber mat and TP at a temperature not lower than the melting point or softening point of the TP in order that the glass fiber mat may be sufficiently impregnated with the TP.
On the other hand, processes for the manufacture of molded articles which comprise heating such a sheet at a temperature which is higher than the melting point or softening point of the TP but lower than its decomposition point and press-molding the same in a matched metal mold held at a temperature below said melting point or softening point (the so-called stamp-molding technique) were proposed in Japanese Patent Publication No. 58-34292 and Japanese Kokai Patent Publication No. 62-92831.
For the production of structural parts, not only high strength and rigidity requirements but also a high heat resistance requirement must be fulfilled. From these considerations, in the heating and compression stage in the manufacturing process for a composite thermoplastic sheet using a glass fiber mat and TP, a high temperature of not less than 200.degree. C. is generally applied. Furthermore, in the step-molding stage in the production of molded articles from a thermoplastic sheet, the sheet is more often than not subjected to a temperature over 200.degree. C.
However, when a high temperature not lower than 200.degree. C. is used in the manufacture of a thermoplastic sheet or in the stamp-molding process, the glass fiber segments are usually discolored by heat. For example, the thermoplastic sheet incorporating a chopped strand mat, which is generally employed for the manufacture of fiber-reinforced plastic products by the hand layup technique (the glass chopped strand mat fibricated using an unsaturated alkyd resin as the binder) and any molded article manufactured by the stamp-molding of such thermoplastic sheet have generally been exposed to high temperatures over 200.degree. C. As a result, both the thermoplastic sheet and the finished article betray yellow-brown discoloration along the glass fibers, thus detracting from the surface appearance of the products. Moreover, even in the production of colored articles, the above-mentioned discoloration causes a distinct demarcation between the area where the glass fiber is present and the adjacent area so that the overall appearance of the article is made unsettled, thus detracting from the market value of the product. The degree of this discoloration tends to be higher when the treating temperature is higher and/or the treatment time is prolonged.
When a chopped strand mat of wholly-aromatic polyester fiber is used, too, the treatment is carried out at a temperature not lower than 200.degree. C. and, consequently, the polyester fiber is discolored. Therefore, the stamp-molded article from the mat presents a generally dull unsettled appearance, thus detracting from its aesthetic quality. The chopped strand mat of carbon fiber is also heat-treated at a temperature over 200.degree. C., so that the areas along the fibers are similarly discolored and the final article is also poor in appearance.